The invention relates generally to the field of illuminators suitable for accurate calorimetric work in electronic imaging and traditional photographic design and production environments. More particularly, the invention concerns illuminators using solid-state emitters and having independent control of both the output spectral characteristic and power level.
Illumination sources are used in the digital still camera (DSC) development and production environments to support a number of different tests and/or calibrations. For example, it is usually necessary to set the relative gains of the DSC""s various color channels to achieve equal channel responses for a given illuminant color temperature. This procedure is often referred to as xe2x80x9cwhite-balancingxe2x80x9d the camera. It is important to use an illumination source during this adjustment procedure that accurately mimics the spectral characteristics of the illumination condition under which the camera will actually be used. Examples of white-balance settings that may be selectable on a DSC are daylight, fluorescent, tungsten, and flash. The term xe2x80x98illuminatorxe2x80x99 and xe2x80x98illumination sourcexe2x80x99 will be used interchangeably throughout this document and should be understood to mean the same thing.
In addition to accurately reproducing a particular illumination spectral power distribution (SPD) characteristic, an illuminator may also be called upon to provide an adjustable output level which does not, in turn, impact the spectral shape of the output. Some examples of the need for this capability are the testing or calibration of a DSC exposure control system or the mapping of the linearity of an imager or DSC tone-scale. This capability is also useful for determining the density versus log-exposure characteristic of photographic films. Usually, the only way to adjust the output level of traditional illumination sources, without changing their spectral shape, is to employ either an adjustable mechanical aperture or a series of selectable neutral-density filters.
Illuminators are used in conjunction with either reflective or transmissive types of color charts to perform calorimetric calibration of a DSC. This procedure requires not only an illuminator with a high-quality spectral output but one whose output spectral characteristics are stable over time. In addition, the spectral characteristics of the color chart must also be stable over time Unfortunately, the aging characteristic of most lamps requires frequent illuminator recalibration in order to maintain accurate results. Physical color charts are often expensive to fabricate and suffer from fading effects that require periodic re-measurement. In addition, it is sometimes difficult to provide uniform illumination over an entire chart and this non-uniformity will impact the DSC calibration. It will be shown later that the present invention allows the color chart to be eliminated entirely, thereby reducing costs and improving calibration accuracy.
One popular illuminator technology is the quartz-tungsten-halogen (QTH) lamp. QTH illuminators typically operate at a color temperature between 2800 and 3200 Kelvins although higher color temperatures can be achieved by a QTH illuminator using appropriate color conversion filters between the illuminator and the DSC. The QTH illuminator is popular because of its relatively low cost and it""s smooth spectral characteristic. Unfortunately, the QTH lamp technology also has many disadvantages. QTH lamps age quickly and must typically be replaced at intervals ranging from 40 to 200 hours depending on how the lamp is operated. Calibration of the lamp must be checked frequently, usually once or twice a day in a production environment, to ensure consistent product quality. QTH lamps also output a significant amount of energy in the infra-red (IR) region and this requires the use of expensive, heat resistant glass filters to block this unwanted energy from the illuminator output.
Halide-metal-iodide (HMI) and xenon lamps are a popular choice for simulating daylight illumination because of their higher correlated-color temperatures. These lamp types also suffer the disadvantages of aging and high output in the form of IR and heat. Both lamp types exhibit some amount of discrete line spectra that may be objectionable for some applications. Xenon lamps also suffer from xe2x80x9carc wanderxe2x80x9d which can affect spatial and temporal uniformity of the illuminator output.
Fluorescent lamps are available with correlated-color temperature ratings suitable for simulation of indoor incandescent as well as outdoor lighting conditions and exhibit a useful life of several thousand hours. One of the disadvantages of fluorescent lamps is the discrete line structure introduced into the SPD characteristic by the Mercury vapor used to excite the phosphors. The mercury line structure is modulated by the AC waveform of the lamp drive current and this can cause a potentially undesirable variation in temporal output of the lamp.
All of the illumination technologies described thus far require a warm up period before their output characteristics stabilize and, typically, this warm up period is on the order of 30 minutes. Even fluorescent lamps, which ignite quickly, require that the glass envelope reach a certain temperature for optimum operation. Because of this warm up characteristic, these types of illuminators must be left in the on-state and a shutter mechanism must be used in conjunction with the illuminator to perform DSC or imager measurements in the dark. The shutter mechanism can be quite expensive when it is also required to accurately control the exposure time on the device or film under test.
In recent years, light-emitting diode (LED) technology has advanced to the point where these devices are now a viable alternative to the traditional illumination technologies used in many applications. LEDs exhibit a turn-on time on the order of 100 nanoseconds, obviating the need for a separate shutter mechanism. Furthermore, because of their stable spectral characteristics, LEDs can be used in conjunction with pulse-width modulation (PWM) to accurately vary output level, thereby eliminating the need for neutral density filters. Appropriate application of PWM to a set of LEDs having a plurality of different peak wavelengths allows the synthesis of different SPDs, thereby eliminating the need for conversion filters.
U.S. Pat. No. 6,127,783 issued Oct. 3, 2000 to Pashley et al., and entitled, xe2x80x9cLED Luminaire With Electronically Adjusted Color Balance,xe2x80x9d describes a luminaire constructed from red, green, and blue LEDs to produce light of different colors based on the mixing together of suitable amounts of the three LED types. The Pashley method relies on the tri-chromatic color theory wherein three color xe2x80x9cprimariesxe2x80x9d with suitable chromaticities in the Commission Internationale De L""Eclairage (CIE) x-y chromaticity diagram are used to create any color whose chromaticity falls within the triangular gamut defined by the chromaticities of the three primaries. A good discussion of tri-chromatic theory and use of the CIE chromaticity diagram can be found in the book Measuring Colour by R. W. G. Hunt (ISBN 0-470-20986-0) in sections 2.4 and 3.3, respectively. Pashley controls the current through the individual LED color channels to adjust the luminous output of each color channel and this has the desired effect of changing the chromaticity of the overall color mixture. The Pashley method suffers from a number of drawbacks when considered in the context of DSC or imager testing and calibration. First, the use of three narrow-band primaries, such as those produced by LEDs, may be inadequate for critical DSC calorimetric calibration because the DSC cannot discern colors in the same way as the human visual system does. This problem arises due to the fact that the spectral sensitivities of the DSC are not color-matching functions (CMFs), i.e. they are not directly related to the spectral sensitivities of the human visual system. During DSC calorimetric calibration, it is therefore important to present the DSC with the actual SPDs of important colors such as grass, skin tones, blue sky, etc. instead of just any SPD which produces the same chromaticity for these colors. It is well known to those skilled in the art of Color Science that a particular chromaticity can be achieved by any one of an infinite number of different SPDs, therefore, the setting of a particular chromaticity using this method does not guarantee the desired SPD for a particular color. The relationship between the LED intensity and forward current is often linear over a reasonable range of operating current so that current control would seem to be an obvious method for adjusting the LED output. Unfortunately, the spectral characteristics of an LED change slightly as a function of forward current making this an unacceptable control method for critical applications.
Another existing illumination development is manufactured by Gamma Scientific, located at 8581 Aero Drive in San Diego, Calif. Gamma Scientific produces a model RS-5 Digital Light Source System(trademark) which is based substantially on the method taught by Pashley. The minimum configuration of the RS-5 utilizes a single model 42200 series LED optical head which comprises a plurality of LEDs of a single type as well as a model 21750 power supply/controller. The minimum RS-5 system varies the optical output power by varying the current supplied to the LEDs. A photodiode feedback circuit, built in to the optical head, is used in conjunction with calibration data to ensure an accurate output level. Gamma Scientific also produces a system comprising four RS-5 systems as described above. Each of the RS-5 systems has a different LED wavelength, in conjunction with a 12-inch integrating sphere for combining the optical outputs from the four LED heads and a personal computer with control software for mixing the primaries according to the user""s desire. It is this latter configuration that allows for the implementation of the Pashley method except with four primaries instead of three.
Further, Gamma Scientific has begun marketing the model RS-5M Programmable Color-Tunable Source(trademark) in addition to the previously described RS-5 Digital Light Source System(trademark). This system comprises an integrating cavity that accepts up to eight standard LED optical heads and a companion control box. One potential drawback to this system is the expense of using the individual LED optical heads. Another potential drawback is the use of too few LED wavelengths to ensure a good spectral match when synthesizing a desired SPD.
U.S. Pat. No. 6,205,244 issued Mar. 20, 2001 to Bawolek, et al., and entitled, xe2x80x9cMethod For Imager Device Color Calibration Utilizing Light-Emitting Diodes Or Other Spectral Light Sources,xe2x80x9d describes several methods of using a plurality of LEDs to colorimetrically calibrate an image capture device. The Bawolek method of most relevance to the present invention involves controlling the relative output power of each of the individual LEDs in order to synthesize the resulting CIE XYZ tristimulus values for each color patch of a Macbeth Color Checker(copyright) under CIE illuminant D65. This is similar to the Pashley method except that more than three primaries are used. Each CIE XYZ triplet as used by Bawolek can be mathematically transformed to a corresponding CIE x-y chromaticity as used by Pashley. The Bawolek method suffers from the same potential problem as the Pashley approach in that the goal is to use the LEDs to synthesize the same XYZ tristimulus value (or x-y chromaticity) for each color patch as would have been obtained from the actual color patch illuminated by CIE illuminant D65. This approach does not guarantee that the SPD for each color patch/illuminant combination will be accurately simulated by the LEDs and the resulting colorimetric calibration will be tainted by the effects of metamerism due to differences between the DSC spectral sensitivities and those of the human visual system.
U.S. Pat. No. 5,982,957 issued Nov. 9, 1999, to Decaro et al., and entitled, xe2x80x9cScanner Illumination,xe2x80x9d describes the construction of an LED illumination system for a film scanner based on the use of a plurality of sets of LEDs having many different wavelengths. In one embodiment, the LEDs are mounted to a single circuit board which is affixed to a concentrator cone which, in turn, is part of a spherical integrating cavity. The purpose of the concentrator cone is to guide the optical energy from the LEDs into the integrating cavity. In another embodiment, multiple circuit boards and companion concentrator cones are used in conjunction with a spherical integrating cavity. The control method for mixing the individual LED channel outputs uses pulse-width modulation with the LEDs operating at a fixed, predetermined current when they are in the on state. This method ensures that the SPD characteristics of the LEDs remain constant. Decaro introduces the concept of spectral matching versus chromaticity matching in order to compensate for the unique characteristics of different manufacturer""s film types. Although the DeCaro spectral matching approach is preferred for use in testing and calibration of image capture devices, he has restricted it""s scope of application to film scanners alone. Use of a concentrator cone to direct the LED energy into the integrating cavity suffers from a couple of drawbacks which are overcome by the current invention. In particular, the physical size required by the concentrator cone prevents a compact illuminator design and the concentrator cone allows first-strike energy which enters the integrating cavity to hit almost everywhere within the cavity thereby reducing the uniformity of the illuminator output.
Therefore, a need persists in the art for an illuminator and method of making same that provides for independent control of spectral characteristics and power level for accurate calibration of image capturing devices.
It is, therefore, an object of the present invention to provide an illuminator for accurate testing and calibration of image capture devices.
It is another object of the invention to provide an illuminator that displays independent control of both output spectral characteristics and power level.
It is yet another object of the invention to provide an illuminator that eliminates the requirement of warm-up time prior to use.
Still another object of the invention is to provide a method of making an illuminator that enables accurate testing and calibration of image capturing devices.
To accomplish these and other objects, features, and advantages of the invention, there is provided, in one aspect of the invention, an illuminator comprising an integrating chamber having a first port for receiving energy and a second port arranged for transmitting energy received through the first port. Energy emitting means is arranged in the first port for directing a beam of energy into the integrating chamber. The energy emitting means is supported so that emitted energy is directed along a predetermined optical path in the integrating chamber. Means for filtering energy of a predetermined wavelength is arranged between the energy emitting means and the integrating chamber. A controller is operably associated with the energy emitting means for receiving user commands and then transmitting the user commands to the energy emitting means thereby enabling the energy emitting means to controllably emit energy.
In another aspect of the invention, a method of making an illuminator includes the step of providing an integrating chamber having a first port for receiving energy and a second port arranged for transmitting energy received through the first port. Energy emitting means are arranged in the first port for directing a beam of energy into the integrating chamber. The energy emitting means are supported so that emitted energy is directed along a predetermined optical path in the integrating chamber. Energy of a predetermined wavelength is filtered from entering the integrating chamber by use of a filtering means. A controller operably associated with the energy emitting means receives user commands and then transmits the user commands to the energy emitting means thereby enabling the energy emitting means to controllably emit energy.
The present invention, therefore, has numerous advantages over current developments, including: lower operating costs associated with the use of solid state emitting means that further obviates the need for frequent bulb replacements; the elimination of frequent calibrations since the spectral characteristics of the LED emitters are stable; the ability to vary the spectral characteristics and power level according to the needs of the source; and, the elimination of the need for a physical color target in order to calibrate the image capturing device.